Geothermal energy is energy in the form of heat within the earth's interior, which is tapped by geothermal wells. Since the earth's interior is extremely hot, there is an enormous potential energy supply in this heat, but there have been, and remain, many technical and economic challenges in optimizing the tapping of this energy source. The use of geothermal energy as an energy source, nonetheless, has gained in importance as other energy sources become less abundant and more expensive. Depending upon the salt content and application, geothermal fluids may be used directly or through a secondary fluid cycle.
Geothermal energy moves towards the earth's surface by thermal conduction through solid rock. Thermal energy can also be transmitted towards the earth's surface by movement of molten rock or by circulation of fluid (H.sub.2 O as steam or water) through interconnected fractures and pores, which may provide heat reservoirs closer to the surface and thus a site more accessible to drilling for wells to tap geothermal energy. Geothermal wells are in any instance relatively deep wells.
Natural geothermal reservoirs, on which many commercial geothermal wells are located, are comprised of volumes of rock at high temperatures (up to about 350.degree. C. or 622.degree. F.) and often also of high porosity and high permeability to fluids. Wells are drilled into such a reservoir and the thermal energy in the rock is transferred by conduction to a fluid (H.sub.2 O as water or steam), which subsequently flows to the well and then up to the earth's surface. In areas where the rock has a low porosity and permeability, it must be artificially fractured by means of explosives or hydrofracturing to provide a network of such fractures.
The thermal fluid within the fractures and pores of a reservoir may be almost entirely in a liquid state, which liquid state exists at temperatures much higher than the boiling point of water at atmospheric pressure because of the high pressure of overlying water. Such a reservoir is referred to as a liquid-dominated, or water-dominated, reservoir. When the thermal fluid within larger fractures and pores is in the form of steam, the reservoir is referred to as a vapor-dominated reservoir. A liquid-dominated reservoir may produce either water or a mixture of water and steam. A vapor-dominated reservoir routinely produces only steam, and in most instances the produced steam is super-heated steam. Water-dominated reservoirs are by far the more common type of reservoir.
In the geothermal production of electricity from a water-dominated reservoir, the pressurized hot water produced from a well is flashed to a lower pressure at the earth's surface, converting the water partly to steam, and this steam is used to drive a conventional turbine-generator set. In a relatively rare vapor-dominated reservoir, the superheated steam may be piped directly to the turbine without the separation of water. Direct use of geothermal energy as a source of heat is practiced in some geographic locations, but the predominant usage remains in the conversion of geothermal energy to electric energy.
Many geothermal wells for the production of electricity (a common use of geothermal energy) are water-dominated hydrothermal convection systems characterized by the circulation of surface water, including wastewaters and/or condensates downhole). The driving force of the convection systems is gravity, the cold downward-moving recharge water being much denser than the heated, upward-moving thermal water. The technique of reinjection of wastewaters or condensates back into the wells may be used for a number of reasons, including avoidance of surface disposal of such waters which may contain pollutants. Selective injection or reinjection of water into the thermal system may help to retain aquifer pressures and to extract more geothermal energy from the rock than is possible when fresh geothermal water is itself the main produced fluid. The produced fluid is either magmatic (released from solidifying magma), meteoric (rain and snow), or a mixture of the two, and may be fresh or reinjected or a mixture of the two.
Geothermal steam is generally used as the energy source, regardless of whether the produced fluid is steam, or partly steam, or the produced fluid is water which is partly converted to steam flash evaporation. Geothermal steam is used in power generation, heating and electrical processes. Geothermal steam temperatures range from about 185.degree. C. to about 370.degree. C. (about 365.degree. F. to about 700.degree. F.), and have a salinity from less than 1000 ppm to several hundred thousands parts per million, and a content of noncondensable gases up to about 6 percent. Geothermal steam power stations generating up to 250 MW have been built.
Mineral deposition is a major problem under the severe conditions encountered in the production of geothermal energy and can be a factor limiting the development of geothermal fields. Mineral deposition from the boiling geothermal fluid of a water-dominated reservoir is particularly a problem. Such mineral deposit problems are commonly the formation of calcium carbonate scale in wells or in the rock, although problems arising from other types of scale are known, for instance silica scale (mainly in re-injection wells). The scale-deposit problems are presently being countered primarily by the down-hole addition of scale inhibitor chemicals, although other techniques, such as acidizing the produced fluids, adding carbon dioxide under pressure thereto, and mechanical methods have been explored.
Scale inhibitors are routinely delivered downhole through long feed lines or tubings that run down through the well through which the hot produced fluids are moving upwardly. Under these conditions scale known inhibitors are extremely corrosive to ferrous metal feed lines, and it is well understood in the field of geothermal wells that the corrosive effect of scale inhibitors on the internal walls of feed lines is the primary cause of feed-line deterioration. Such feed lines are frequently now formed of tubes of highly resistant, and very expensive, alloys to resist the corrosion attack from within. These alloy tubes may even then be encased within mild steel tubing to prevent stress corrosion cracking and abrasion against the wall and/or the wellhead crown valve. The use of just the outer mild steel material is not possible presently because corrosion failure after a just a few days of use for delivering present scale inhibitors would be expected.
It is an object of the present invention to provide a scale inhibitor, particularly a scale inhibitor for geothermal wells, of reduced corrosivity. It is an object of the present invention to provide a scale inhibitor, particularly a scale inhibitor for geothermal wells, of such reduced corrosivity that it is relatively noncorrosive to mild steel. It is an object of the present invention to provide a method for delivering scale inhibitors downhole whereby the corrosivity of such scale inhibitors is reduced. These and other objects of the present invention are described in more detail below, as are the conditions promoting the inhibitor corrosivity problem and the problems created by such corrosivity.